Borehole radar systems typically operate with electromagnetic waves in the short and ultrashort wave ranges. For these ranges, the available borehole diameter is very small relative to the wavelength. Heretofore, this fact has prevented the use of antenna arrangements which are usable so as to have a horizontal directional action. In the customary probes, only dipole antennae are used which, in the horizontal plane, have omnidirectional characteristics, and folded dipoles can also be used as shown in U.S. Pat. No. 3,286,163. Insofar as complicated antennae are used, such as for example, the Yagi arrangement shown in FIG. 4 of U.S. Pat. No. 3,286,163, it is a prerequisite that the parts of the antenna are opened up in a telescopic fashion so that they project beyond the outer cylindrical surface of the probe. Such an antenna arrangement presupposes boreholes having diameters which exceed the customary dimensions so that its use will not be limited to enlarged areas in a standard borehole.
In the vertical direction, a determination of the direction of reception of the radar signals received can be made because of the fact that measurements are made on a series of vertically successive measuring points. Thus, for the vertical resolution of direction, a directional antenna is not required although it could easily be constructed. It should be furthermore noted that for physical reasons no effective directional converging or focusing can be achieved with electric field sensors in narrow boreholes because a determination of the direction can be deduced only from the differential information of at least two sensors which must be spaced apart by a significant and determinable portion of one wavelength in the wave field.
Loop antennae have been used successfully for some time as so-called direction finding loops or direction receiving installations in radio engineering generally as discussed, for example, in Handbuch fur Hochfrequenz- und Elektro-Techniker (Manual for High Frequency-and Electrotechnicians), Vol. II, 1953, pp 489 and 490. Because of the relatively low induced voltages, such loops are made, without exception, as selective arrangements for the narrow band reception of selected carrier frequencies. The induced voltage in a loop antenna is proportional to the surface of the loop, the frequency and the cosine of the angle of incidence of the wave front. Contrary to the omnidirectional pattern of a rod or dipole antenna, loop antennae have a double-circle response pattern in a horizontal direction with two distinct zero locations. With a properly adapted combination of a dipole and of a loop antenna, a cardioid pattern can be achieved with only one pole, a so-called zero position.
In order to determine the direction of incidence, the so-called direction finding loop is rotated around the vertical axis until a zero point is determined. This so-called minimum direction finding delivers the most precise results because of the steep characteristic of the zero points. When a rotatable direction-finding loop cannot be erected for reasons of construction or electricity, it is customary today to use a fixed, crossed loop antenna together with an electrical goniometer. When using such a goniometer, the field of the receiving crossed loop is reproduced by an arrangement of two coils positioned at right angles to each other and at the inside a rotary coil which serves as an exploring coil. The rotation of the exploring coil simulates a rotation of the loop antenna arrangement.
These known loop antenna arrangements can be advantageously used for the determination of the direction of incidence of discrete carrier frequencies. The combination used for the unequivocal determination of direction of a directional finding loop with an auxiliary antenna for circular reception requires very careful synchronization of the system and presupposes time-stable carrier frequencies.
These arrangements and processes, which have been known for a long time in radio engineering, could not heretofore be used for borehole measuring processes because of the lack of space.